1. Field of the Invention
The present invention relates to a method and a device for manufacturing single crystals by solidification of a liquid placed in the presence of a single-crystal solid.
2. Discussion of the Related Art
An application of the present invention relates to a device for manufacturing single crystals comprising a crucible in which are placed a seed of the single crystal to be formed and a liquid phase of the crystal. The liquid is progressively cooled down from the region close to the seed. The liquid first solidifies close to the seed, then the liquid-solid interface displaces inside of the crucible until the solidification is total. Since the newly-formed solid reproduces the crystal structure of the previously-formed adjacent solid, the seed imposes from close to close its crystal structure to the entire content of the crucible.
A problem of such a method is however linked to the differences in thermal expansion between the crucible and the crystal that it contains. Indeed, if the crucible contracts more than the crystal during the cooling, the single crystal risks being damaged or even fractured. It is then further difficult to extract the single crystal from the crucible, which must generally be destroyed. If the crystal contracts more than the crucible, the crystal does not necessarily remain intact either since it generally tends to adhere to the crucible after the solidification, and thus risks undergoing tensile stress on cooling.
FIG. 1 schematically shows a device for manufacturing single crystals which does not exhibit the previously-mentioned disadvantage. The device comprises a cylindrical crucible 1 containing a single-crystal solid phase 2 at its lower portion and a liquid phase 3 to be solidified above an interface 4 with solid phase 2. Liquid phase 3 bathes the wall of crucible 1, while solid phase 2 is separated from the wall of crucible 1 by an interstice 5. An upper duct 6 emerges into crucible 1 above the free surface of liquid phase 3, and a lower duct 7 also emerges into crucible 1, at the level of interstice 5. Ducts 6 and 7 join at the level of a system 8 capable of creating a differential pressure so that the pressure injected into lower duct 7 is greater than that of upper duct 6 by a value substantially equal to the hydrostatic pressure of liquid phase 3, that is, to the pressure generated by the height of liquid column 3. Interstice 5 spontaneously appears in such conditions when the crystal solidifies, interface 4 connecting to the wall of crucible 1 by a meniscus 10 above interstice 5. Such an example of a device for manufacturing single crystals is described in French patent application 2757184 filed by the Commissariat à l'Energie Atomique.
FIG. 2 shows a partial cut-away view of a variation of the single crystal manufacturing device of FIG. 1 in which crucible 11 is formed of a tightly-sealed tube placed on a support 12. A single-crystal solid phase 13 is attached to support 12 in crucible 11 and is covered with a liquid phase 14. A first furnace 15 surrounds crucible 11 substantially opposite to liquid phase 14. A second furnace 16 surrounds crucible 11 substantially opposite to solid phase 13. First and second furnaces 15, 16 impose a local temperature gradient in crucible 11 causing the solidification of liquid phase 14 at the level of the liquid-solid interface. Along the solidification of liquid phase 14, tight crucible 11 is displaced by motion of support 12 so that the liquid-solid interface is substantially fixed with respect to first and second furnaces 15, 16 and permanently is at the level of the temperature gradient. An interstice 17 exists between solid phase 13 and crucible 11. Liquid phase 14 connects solid phase 13 to crucible 11 by a meniscus 18. Interstice 17 is filled with a neutral gas which maintains meniscus 18. The pressure of the neutral gas is set via a third furnace 19 which heats up the lower portion of crucible 11. Such an example of a single crystal manufacturing device is described in French patent application 2806100 filed by the Commissariat à l'Energie Atomique.
The previously-described single crystal manufacturing devices enable avoiding the contact between the crucible and the single crystal. However, the use of such devices is difficult. Indeed, the difference between the pressures applied on meniscus 10, 18, and on the free surface of the liquid phase must decrease along the crystal growth since the height of liquid column 3, 14, and thus, the resulting hydrostatic pressure, decreases. Further, the component(s) forming liquid phase 3, 14 have vapor tensions that may be high. This may translate as non-negligible gas exchanges between liquid phase 3, 14 and the environing gas, at the level of meniscus 10, 18 and at the level of the free surface of liquid phase 3, 14. Such exchanges complicate the control of the single crystal manufacturing method since they tend to modify the difference between the pressures applied on meniscus 10, 18 and on the free surface of liquid phase 3, 14. Further, for a single crystal comprising several components, such gas exchanges tend to modify the proportion of the components in liquid phase 3, 14. The obtained single crystal can then not have the desired composition. It is thus necessary to take into account the vapor tensions of the components of liquid phase 3, 14 to determine the pressure difference to be applied, which appears to be very delicate in practice, or even contrary to the obtaining of interstice 5.